1. Field of the Invention
The present invention relates a phase interpolator, and especially a digitally-controlled phase interpolator.
2. Discussion of the Related Art
Phase interpolators find applications, in particular, in the field of digital frequency synthesis.
There mainly exist two types of phase interpolators. A phase interpolator of the first type receives two phase-shifted initial signals having the same waveform and provides, by phase interpolation of the initial signals, a signal having a waveform similar to the initial signals and having a phase ranging between the phases of the two initial signals. A phase interpolator of the first type thus requires, for each signal to be provided, two initial signals such that the signal to be provided has a phase interposed between those of the initial signals, so that the interpolation operation can be performed. It is thus necessary to have many initial signals. Thereby, such a phase interpolator generally has a complex structure, takes up a significant surface area when made in integrated form, and has a high power consumption.
A phase interpolator of the second type implements delay elements. The delay elements may be so-called simple delay elements, for example, inverters. The operation of a simple delay element is based on the propagation time of a signal between the input and the output of the delay element. This time generally depends on parameters such as the capacitance or the supply current of the delay element. A phase interpolator with simple delay elements is generally highly sensitive to the initial noise and has operating characteristics which depend on the interpolator manufacturing process and on temperature. It is further difficult to form a phase interpolator with simple delay elements which is digitally-controlled. The delay elements may be formed from synchronized oscillators. An example of a phase interpolator comprising synchronized oscillators is described in French patent application 2841406 filed by STMicroelectronics. Such an interpolator advantageously enables avoiding adding noise to the phase of the supplied signal. However, currently-available synchronized oscillator phase interpolators generally have a complex structure, especially due to the use of a large number of synchronized oscillators.